1. Field of the Invention
The present invention relates to an amplitude-modulated signal receiving circuit for receiving and demodulating AM (here signifying double sideband emitted carrier amplitude modulation) signals that are widely employed in the medium-frequency (medium wave) and high-frequency (short wave) bands. The invention relates more particularly to techniques for improving the quality of the demodulated signal.
2. Description of Related Art
Given the level of technology when practical AM receivers became available, and given the requirement at that time for a simple yet economic demodulation scheme that did not need an elaborate demodulation circuit, the demodulation method adopted was envelope detection (demodulation), which is the simplest method.
Nowadays, AM broadcasting is the most representative example of the utilization of amplitude modulation. Because a great many people throughout the world can receive and make use of AM broadcasting, there are a large number of receivers in existence. Historically speaking, the above-mentioned detection (demodulation) method was certainly the best available when AM broadcasting became practical.
Moreover, because an AM signal can be demodulated by means of envelope detection, great important is attached, in its application to aircraft radio, to the characteristic that such demodulation is largely independent of the frequency stability of the radio equipment, and hence this technique is still in use today, even though the quality of the demodulated signal is poor.
The following problems have been encountered with conventional AM signal demodulation techniques.    1. Due to amplitude and phase fluctuations that occur in the propagation path through which an AM signal propagates, it is difficult to obtain high-quality demodulated signals using conventional demodulation techniques. Sources of such amplitude and phase fluctuations include, for example, multiplicative disturbance caused by fading, etc., and power source noise, fluorescent light noise and city noise.    2. Amplitude modulation technology has a long history, and the technical level prevailing when it first came into practical use was such that modulation schemes did not employ measures to remove these disturbances and thereby obtain high-quality demodulated signals.    3. An AM broadcast wave is transmitted with 400 Hz referenced to 0 dB, and with the 50 Hz to 7500 Hz range held within 1.5 dB to −3 dB. However, the bandpass characteristics of the receiver are less flat than those of the transmitted wave, and even if these bandpass characteristics are adjusted for sound quality after reception and demodulation, audio quality cannot be improved. The main reason for this is that conventional demodulation technology provides no means for removing the multiplicative noise that affects an AM signal. Moreover, because the signal is susceptible to additive noise as well, AM receivers are designed to have single-peak receiving bandpass characteristics so as to be more audible. Accordingly, despite the need for flat frequency characteristics over a sufficiently wide bandwidth, existing receivers cannot guarantee really excellent sound quality for audio sources such as music.    4. Although automatic gain control circuits are used to automatically control the received input power level, these circuits are not sufficient to improve the quality of the demodulated signal.